Beilina Alexandra, Tassone Flora, Schwartz Philip H, Sahota Parminder, Hagerman Paul J
Department of Biological Chemistry, University of California, Davis, School of Medicine, Davis, CA 95616, USA.
Hum Mol Genet. 2004 Mar 1;13(5):543-9. doi: 10.1093/hmg/ddh053. Epub 2004 Jan 13.
Fragile X syndrome, the most common form of mental impairment, is caused by expansion of a (CGG)n trinucleotide repeat element located in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene. Repeat expansion is known to influence both transcription and translation; however, the mechanisms by which the CGG element exerts its effects are not known. In the current work, we have utilized 5'-RLM-RACE to examine the influence of CGG repeat number on the utilization of transcription start sites in normal (n<55) and premutation (54<n<200) cell lines of both non-neural (lymphoblastoid) and neural (primary astrocyte) origin. Our results demonstrate that, in both neural and non-neural cells, transcription of the FMR1 gene is initiated from several transcription start sites within a approximately 50 nt region that lies approximately 130 nt upstream of the CGG repeat element. For normal alleles, most transcripts initiate from the downstream-most start site, close to the single position identified previously. Surprisingly, as the size of the CGG repeat expands into the premutation range, initiation shifts to the upstream sites, suggesting that the CGG element may act as a downstream enhancer/modulator of transcription. The shift in start site selection for both neural and non-neural cells indicates that the effect is general. Furthermore, the correspondence between start site utilization and the degree of elevation of FMR1 mRNA suggests that a substantial fraction of the increased message in the premutation range may derive from the upstream start sites.
脆性X综合征是最常见的智力障碍形式，由位于脆性X智力低下1（FMR1）基因5'非翻译区的（CGG）n三核苷酸重复元件的扩增引起。已知重复扩增会影响转录和翻译；然而，CGG元件发挥其作用的机制尚不清楚。在当前的研究中，我们利用5'-RLM-RACE来研究CGG重复数对非神经（淋巴母细胞样）和神经（原代星形胶质细胞）来源的正常（n<55）和前突变（54<n<200）细胞系中转录起始位点利用的影响。我们的结果表明，在神经细胞和非神经细胞中，FMR1基因的转录都从CGG重复元件上游约130 nt处一个约50 nt区域内的几个转录起始位点开始。对于正常等位基因，大多数转录本从最下游的起始位点开始，接近先前确定的单个位置。令人惊讶的是，随着CGG重复序列的大小扩展到前突变范围，起始位点转移到上游位点，这表明CGG元件可能作为转录的下游增强子/调节剂。神经细胞和非神经细胞起始位点选择的变化表明这种影响是普遍的。此外，起始位点利用与FMR1 mRNA升高程度之间的对应关系表明，前突变范围内增加的信息中很大一部分可能来自上游起始位点。
